1. Field of the Invention
The present invention relates in general to differential pressure transducers and in particular to a new and useful differential pressure transducer using a rigid beam.
2. Description of the Related Art
Presently, differential pressure (dP) transducers generally contain a displacement sensor coupled between two thin diaphragms. The two coupled diaphragms perform a mechanical subtraction of the pressures on them and the sensor measures their net motion relative to the transducer body to determine differential pressure. In order to prevent diaphragm rupture while maintaining the desired sensitivity to differential pressure, the volume between the diaphragms, including the sensor, is filled with a hydraulic fill fluid. When process line pressure is presented to one side of each diaphragm, the fill fluid is pressurized to the line pressure. If the boundary of the volume between the diaphragms, including the diaphragms, electrical feedthroughs and fill/bleed ports, is not totally sealed small leaks of fill fluid will occur and will cause unacceptable increases in response time, sensor output drift and transducer non-linearity with pressure. In some cases, these changes may not be readily detected when the transducer is in service because the transducer output may remain stable at constant dP. The leaking of fill fluid from these known dP transducers is a problem which is well documented.
Another inadequacy of fluid-filled dP transducers is the static pressure effect. A dP transducer as described should output a value of zero when the same process pressure is applied to both diaphragms. However, the static pressure causes the fill fluid to be pressurized, resulting in distortions of the transducer body. These distortions cause relative motions between the diaphragms and body resulting in static pressure effect on zero, and also produce radial forces on the diaphragms which change their effective stiffness and cause static pressure effects on span. In addition, the displacement sensor is exposed to the fill-fluid pressure environment adding to the static pressure effects on both zero and span. In applications involving static pressures of several thousand psig or greater, the requirement for a stable zero and span over the allowable range of static pressures is difficult to achieve in practice.
The use of a fill fluid also contributes to degraded performance of a dP transducer when it is operated over a range of temperatures, as is normal in service. The volumetric expansion of liquids with temperature is significantly greater than that of the metals used in construction of the transducer body. Thus, when the temperature of the transducer changes, the volume of the fill fluid changes more than the volume of the body. This results in motion of both the thin diaphragms away from their rest positions, distorting their shape and causing degraded linearity and accuracy. The normal means for limiting this effect is to keep the volume of the fill fluid at an absolute minimum; this means aggravates the effect of leakage because a leak of a given volume is a more significant part of the total fluid volume.
Rather than perform a mechanical subtraction of two large pressures as described above, an alternative approach would be to measure each pressure with a separate gage pressure transducer and then subtract the signals electronically. If the full-scale differential pressure range to be measured is 1000 in H.sub.2 O (37 psi) and the desired accuracy is 0.1% FSI (0.04 psi), then for application at 3000 psig line pressure, a gage pressure transducer is required which has an accuracy of 0.04/3000=0.0013% (1:75,000). Such devices are not commercially available, and not yet achieved with any known technology. Thus, mechanical subtraction of two large pressures is the only alternative measurement approach available with present day technology.
Presently, there is no known system or method for providing a differential pressure transducer which avoids the problems associated with the known devices listed above.